Femtocell channel assignment and power control for improved femtocell coverage and efficient cell search

ABSTRACT

A method and a communication system including femtocells within a macrocell efficiently manage interference between the different femtocells, and between each femtocell and a macrocell. An efficient frequency assignment scheme for the femtocells minimizes interference between a femtocell and a macrocell and among different femtocells using a spectrum-sensing technique carried out by the femtocells. The frequency assignment scheme selects a suitable channel from a set of candidate channels and ensures that the femtocell has an acceptable coverage area even when it is close to the macrocell base station (BS). The frequency assignment scheme favors a co-channel implementation to take advantage of the hand-off and cell search characteristics of the co-channel implementation. In one embodiment, a joint power control and frequency band assignment technique is used, which partitions the coverage area of the macrocell into an inner region, a power control region, and an outer region.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 12/463,307, filed on May 8, 2009, which in turn claims priorityto U.S. Provisional Patent Application No. 61/055,345, filed on May 22,2008 and to U.S. Provisional Patent Application No. 61/073,276, filed onJun. 17, 2008.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communication. Morespecifically, the present invention relates to methods for efficientlyassigning channels to femtocells, taking into account hand-off,interference, coverage area, and power control considerations.

2. Discussion of the Related Art

Because mobile telephones may be used practically everywhere, they arereplacing fixed wired telephones. The article, “UMA and Femtocells:Making FMC Happen” (“Choudhury”), by Partho Choudhury and Deepak Dahuja,“White Paper, December 2007. (available: athttp://www.parthochoudhurv.com/UMAFemto.doc), discloses (a) thatapproximately 30-35% of all voice calls made over a mobile network aremade by mobile subscribers at their homes, and (b) about 35% of videostreaming and broadcasting service uses over cellular wireless networksin 2006 took place while the mobile subscribers are at their homes.

The trend, therefore, is for the mobile telephone to become the primaryor only telephone for an individual subscriber. Furthermore, thearticle, “Femto Cells: Personal Base StationA” (“Airvand”), published byAirvana Inc., White Paper, 2007 (availablehttp://www.airvana.com/files/Femto_Overview_Whitepaper_FINAL_(—)12-July-07.pdf),reveals that those 24 years of age or younger make up to 80% of theirlong distance calls on wireless networks rather than over wirednetworks. However, there is still much to be improved in reliability,voice quality, and cost of today's mobile telephone networks in indoorenvironments. Typically, the mobile telephone service is more costlythan a wired telephone service, and there are dead spots and poorcoverage. These deficiencies result in poor customer experience, thuspreventing the mobile telephone to successfully replace the wiredtelephone as the primary or only telephone for most subscribers.

Choudhury, Airvana, and the article “The Case for Home Base Stations”(“PicoChip”), published by PicoChip Designs Ltd., White Paper, April2007 (availablehttp://www.picochip.com/downloads/27c85c984cd0d348edcffe7413f6ff79/femtocell_wp.pdf)all disclose a new class of base stations (BSs) designed for indoor andpersonal uses, The cells served by these personal BSs have come to beknown as “femtocells.” A femtocell (e.g., the home e-node B (HeNB)defined in the 3GPP standard) enables indoor wireless connectivitythrough existing broadband Internet connections. As described inChoudhury, femtocells are also featured in fixed-mobile convergence(FMC), where the subscribers are provided the ability to switch anactive data/voice call session between home wireless network (e.g.,femtocell) and a mobile network (e.g., a cellular network). As reportedby Choudhury, Airvana and PicoChip, the benefits of femtocells includeimproved indoor coverage, reduced capital and operational expenditure,reduced bandwidth load, reduced power requirements, additional high-endrevenue streams, improved customer royalty, increased average revenueper user, compatibility with existing handsets (without requiringdual-mode terminals), deployment in an operator-owned spectrum, andenhanced emergency services (since the femtocells are location-aware).

Despite these benefits, femtocell technology is still at its infancy. Asidentified in Airvana, the technical issues to be solved include thoserelated to interference management (both between different femtocellsand between the femtocell and the macrocell), efficient hand-offmechanisms, security, scalability, and access control. For example,co-channel implementations of femtocells—where the macrocell network andthe femtocell network share the same frequency band—introduce seriouschallenges. Co-channel deployment of femtocells has desirable hand-offcharacteristics, as a mobile station (MS) may more efficiently scan thecells using the same frequency band compared to identifying the cellsusing other frequency bands, which require band-switching to accomplishthe scanning. However, for distances that are close to the macrocellbase station (mBS), severe interference from the mBS may preventco-channel deployment.

The article, “Effects of User-Deployed, Co-Channel Femtocells on theCall Drop Probability in a Residential Scenario” (“Lester”), by LesterT. W. Ho and Holger Claussen, published in Proc. of IEEE Int. Symp. onPersonal, Indoor and Mobile Radio Communications (PIMRC), pp. 1-5,September 2007, shows that the received signals from the femtocell andthe macrocell in such an implementation have identical power levels atthe border of the macrocell. Thus, without adequate power control, thefemtocell coverage area decreases for those femtocells that are closerto the macrocell BS (mBS). However, when the femtocell coverage areafalls below a certain size, the femtocell does not completely cover auser's premise, which is the preferred coverage area. A differentsolution is desired, under such circumstances.

The article, “Uplink Capacity and Interference Avoidance for Two-TierCellular Network” (“Chandrasekhar”), by Vikram Chandrasekhar and JeffreyG. Andrews, published in Proc. IEEE Global Telecommunications Conference(GLOBECOM), pp. 3322-3326, November 2007, derived and analyzed theuplink (UL) capacity of a co-channel femtocell network coexisting with amacrocell network (i.e., a shared-spectrum network). In a split spectrumnetwork, the femtocell users and the macrocell users use orthogonalsub-channels. While the split spectrum network avoids interferencebetween the macrocell and the different femtocells, the total number ofusers that can be supported is less than a shared spectrum network. In ashared spectrum network, a femtocell may use a sub-channel that isalready used in the macrocell, so long as there is little interferencebetween the femtocell and the portion of the macrocell network where thecommon sub-channel is used. In a co-channel femtocell deployment, an MSneed not scan through multiple frequency bands to search for the cell.

Chadrasekhar suggests using interference avoidance methods to reduce theoutage probability. For example, each macrocell user and each femtocellmay employ time-hopping in order to decrease interference. Further, themacrocell and femtocell may both use a sectored antenna reception forimproving the capacity. Chandrasekhafs analytical/simulation resultsshow that, by using interference avoidance (specifically, time-hoppedcode-division multiple access (TH-CDMA) and sectorized antennas), up toseven times higher femtocell BS (fBS) density can be supported in ashared spectrum network, relative to to a split spectrum network withomnidirectional femtocell antennas. However, sectored antennas may bedifficult to implement at the femtocells (which are necessarily, forpractical considerations, simpler devices than regular BSs). Further, atime-hoping approach increases symbol duration (and hence, decreasesdata rate).

Lester, discussed above, analyzed hand-off probabilities for differentpower configurations at a femtocell. Since the manual cell-planning usedin macrocell networks is not economically practical for femtocells,femtocells typically require auto-configuration capabilities (e.g.,automatic power and cell size configuration). Lester's simulations showthat call drop probabilities can be significantly decreased in aresidential co-channel femtocell deployment through simple pilot poweradaptation mechanisms.

The article, “Performance of Macro- and Co-Channel Femtocells in aHierarchical Cell Cell Structure” (“Clausseri”), by Holger Claussen,published in Proc. of IEEE Int. Symp. on Personal, Indoor and MobileRadio Communications (PIMRC), pp. 1-5, September 2007, discloses asimple power control algorithm for pilots and data signals infemtocells. Simulation results show that the interference to themacrocell network can be minimized through intelligent power controltechniques.

In Lester, Chandrasekhar and Clausen, relatively simple power controlmechanisms are proposed for femtocells, so that thesignal-to-interference ratio (SINR) is equal to 0 dB at the cell edge.However, depending on the distance between the mBS and the fBS, suchpower control strategies may not be effective. For example, as mentionedabove, the maximum transmission power of a co-channel femtocell may notbe sufficient to provide satsifactory coverage when the fBS is close tothe mBS.

The article, “Home NodeB Output Power,” published by Ericsson, 3GPP TSGWorking Working Group 4 meeting (available athttp://www.3gpp.org/ftp/tsg_ran/WG4_Radio/TSGR4_(—)43bis/Docs/),provides a power control scheme which reduces the femtocell transmitpower as the distance between the macrocell BS and the femtocell BSincreases. Under such an arrangement, the macrocell MSs experiencebetter coverage as a result of reduced interference from the femtocell.However, this approach is questionable when a femtocell is either veryclose to or very far away from the macrocell BS.

The article, “Uplink User Capacity in a Multicell CDMA System withHotspot Microcells,” by S. Kishore, L. J. Greenstein, H. V. Poor, and S.C. Schwartz, published in IEEE Trans. On Wireless Communications, vol.5, no. 6, pp. 1333-1342, June 2006, overcomes the near-far effect byincreasing femtocell coverage. Increased femtocell coverage is achievedby allowing an MS close to a femtocell to communicate with the macrocellBS only when the signal quality from the macrocell BS is significantlybetter. This approach increases interference at neighboring femtocells.

The following patent application publications disclose femtocellimplementations: (a) U.S. Patent Application Publication 2007/0183427,“Access Control in Radio Access Network Having Pico Base Stations,” byT. Nylander et al., filed Oct. 3, 2006; (b) U.S. Patent ApplicationPublication 2007/0254620, entitled “Dynamic Building of Monitored Set”,by T. L. E. Lindqvist et al., filed Apr. 28, 2006; and (c) InternationPatent Application Publication WO2006/0139460, entitled “Method andApparatus for Remote Monitoring of Femto Radio Base Stationg”, by J.Vikeberg et al., May 30, 2006. However, none of these patentapplications offers an efficient frequency assignment scheme for afemtocell deployment.

SUMMARY OF THE INVENTION

Femtocells may increase the efficiency and coverage of macro cellularnetworks. Successful femtocell deployment depends on efficientlymanaging both interference among different femtocells and interferencebetween a femtocell and a macrocell.

According to one embodiment of the present invention, an efficientfrequency assignment scheme for femtocells is provided. The frequencyassignment scheme reduces the interference between a femtocell and amacrocell and among different femtocells. Under this method, based onits location and where required, the femtocells perform spectrum-sensingand select suitable channels from a set of candidate channels. Themethod also provides an acceptable femtocell coverage area, even whenthe fBS is close to the mBS (i.e., as determined using a threshold). Thefrequency assignment scheme prefers sharing the frequency band of themacrocell network with the femtocells to take advantage of the desirablehand-off characteristics (i.e., the MS need not scan different frequencybands to search for cells). However, when the interference from the mBSexceeds a threshold, the assignment scheme requires the femtocell to usea different frequency band selected from a set of appropriatefrequencies to ensure an acceptable femtocell coverage area. Thefrequency assignment scheme is applicable to various types offemtocells, such as the Home eNodeB and other personal BSs.

According to another embodiment of the present invention, the frequencyassignment scheme may also provide joint power control and frequencyallocation. Under this embodiment, the frequency assignment scheme ispriority-based, such that the candidate frequency bands are selecteddepending on various parameters, such as the relative locations of thefBS and the mBS, path loss exponents and a frequency reuse factor (N).When the fBS is far away from the mBS, the fBS selects a frequency bandfrom among those used by the mBS, and when the fBS is close to the mBS,a different frequency band is assigned to the fBS to achieve anacceptable coverage area. When the femtocell is located within a powercontrol region, the transmission power of the femtocell is controlled tomaintain a fixed coverage area for the femtocell. Among the femtocells,spectrum-sensing is used to select candidate frequency bands for use, inorder to reduce interference among different femtocells.

The present invention is better understood upon consideration of thedetailed description below in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a macrocell network that has a frequency reuse factorN>1.

FIG. 2 illustrates a frequency band assignment for a network having areuse factor N=3, where no neighboring macrocells use the same frequencyband.

FIG. 3 shows a partition of the macrocell into two regions (i.e., aninner region and an outer region), in accordance with one embodiment ofthe present invention.

FIG. 4 is a flow chart illustrating in greater detail a frequencyassignment framework according to one embodiment of the presentinvention.

FIG. 5 is a flow chart illustrating another femtocell frequencyassignment framework, according to a second embodiment of the presentinvention.

FIG. 6 illustrates a spectrum scanning technique, according to oneembodiment of the present invention.

FIG. 7 shows yet another frequency band assignment framework, inaccordance with one embodiment of the present invention.

FIG. 8 shows, for N=1, another frequency assignment scheme which assignsa separate frequency band to each femtocell, according to one embodimentof the present invention.

FIG. 9 shows a frequency division duplex (FDD) frequency assignmentscheme for a macrocell network, in accordance with one embodiment of thepresent invention.

FIG. 10 shows an example of an actual FDD frequency assignment in anetwork of the type discussed with respect to FIG. 9, according to oneembodiment of the present invention.

FIG. 11 illustrates a network implementing joint power control andfrequency assignment, in accordance with one embodiment of the presentinvention.

FIG. 12 provides an example of frequency band assignments in the regionsof FIG. 11, for both downlink and uplink frequency assignments, inaccordance with one embodiment of the present invention.

FIG. 13 is a block diagram illustrating joint power control andfrequency band assignment in the regions of FIG. 11, in accordance withone embodiment of the present invention.

FIG. 14 is a block diagram illustrating a first approach the fBS may useto determine the region of its location, in accordance with oneembodiment of the present invention.

FIG. 15 is a block diagram illustrating a second approach the fBS mayuse to determine the region of its location, in accordance with oneembodiment of the present invention.

FIG. 16 shows one implementation of a method for refining thecalculation of transmit power, in accordance with one embodiment of thepresent invention.

FIG. 17 shows an examplary message exchange between an fBS and an MS, inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a macrocell network 0 that has a frequency reusefactor N>1. In such a network, N frequency bands are assigned to themacrocells of the network, such that each macrocell is assigned adifferent frequency band than any of its neighboring macrocells. In theexample of FIG. 1, without loss of generality, frequency band F_(i) isassigned to macrocell 0. FIG. 2 illustrates a frequency band assignmentfor a network having reuse factor N=3, where no neighboring macrocellsuse the same frequency band. As shown in FIG. 2, macrocell 4, forexample, is assigned frequency band F₁. Within each frequency band, theusers may be further separated in time, frequency, or code domains. Forexample, in a wideband code division multiple access (WCDMA) network,each user in a macrocell is assigned a different CDMA code, selectedfrom a set of CDMA codes, to minimize interference among users in themacrocell cell. There may be tens or even more femtocells within amacrocell.

As discussed above, a co-channel implementation is preferable for afemtocell, from hand-off and cell search points of view. However,co-channel operation may not always be possible, due to interferencefrom the mBS. FIG. 3 shows a partition of the macrocell into two regions(i.e., inner region 6 and outer region 7), in accordance with oneembodiment of the present invention. Femtocells that are located inouter region 7 may use the same frequency band as one that is used inthe macrocell network. For example, fBSs in outer region 7 and the mBSmay both use frequency band F₁. Under such an arrangement, an MS canexperience easier hand-off and cell search, when switching between thefemtocell and the macrocell, as both cells use the same frequency band.Also, since the fBS is far away from the mBS, there is littleinterference from the mBS. Consequently, the IBS has a sufficientlylarge¹ coverage area. ¹ The coverage area is particularly sensitive toan interference-limited environment. If the interference is weak, i.e.,a noise-limited environment, the coverage area would depend on thenoise-floor. Interference is weak when the fBS is far away from the mBS.Where interference is strong, it is best to use a frequency band that isdifferent from that of the macrocell.

For this detailed description, the coverage area of a fBS is defined forconvenience sake by a contour along which the levels of the receivedpower from the fBS and from the mBS are the same. The present inventionis, however, is not limited by this convention. The present invention isapplicable to situations where the coverage area may be defined in otherways. For example, the coverage area may be defined by a hand-offparameter, such as the cell search initiation threshold or the hand-offexecution threshold. In such a case, the received power levels from thefBS and from the mBS may not be the same. Also, in such other cases,there may be more than one contour that defines the coverage area (e.g.,a contour for an incoming user to the fBS, and another contour for anoutgoing user from the fBS).

Referring to FIG. 3, for fBSs that are located within inner region 6,the power level received from the mBS is high. Therefore, if these fBSsuse the same frequency band as the mBS, their respective coverage areasare small, such as illustrated by fBS₂ in femtocell coverage area 8. Fora reuse factor N>1, the femtocell may use one of the other N−1 bands,that are used by other macrocells and not the current macrocell. Forexample, fBS₃ uses frequency band F₂, which is an orthogonal channel tofrequency band F₁. As interference from other macrocells is expected tobe minimal, the coverage area for the femtocell fBS₃ may be keptsufficiently large.

FIG. 4 is a flow chart that illustrates in greater detail a frequencyassignment framework according to one embodiment of the presentinvention. In FIG. 4, n₁ denotes the path loss exponent (PLE) between anMS and the mBS, n₂ denote the PLE between an MS and the fBS, P₁ denotethe received signal power from the mBS at an MS, and P₂ denote thereceived signal power from the fBS at an MS. Also, (x_(mBS), y_(mBS))and (x_(fBS), y_(fBS)) denote the geographical locations of the mBS andthe fBS, respectively². The values of these parameters are measured atstep 15. At step 20, as the SINR is 0 dB at the border of a femtocellcoverage area (under co-channel implementation), the femtocell coveragearea may be calculated algebraically. One method for calculatingfemtocell coverage area is discussed in further detail below^(3,4).Coverage area A_(fBS) of a co-channel femtocell preferably coverscompletely a user's premises. At step 30, coverage area A_(fBS) iscompared to a threshold value. If the coverage area is greater than thethreshold value, the femtocell can operate in a co-channel manner withthe macrocell. Otherwise, i.e., if the coverage area A_(fBS) is lessthan the threshold value, severe interference is experienced, and thefemtocell should select a different frequency band. While the thresholdvalue may be based only on coverage area considerations, it can alsotake into account other parameter values, such as the monthlysubscription fee charged to the user pays, or a QoS requirement, andother interference considerations. ² The relative locations of the twoBSs with respect to each other provide the distance between them.³ Thepath loss exponents may be obtained from prior measurements performed inthe same environment. Alternatively, it may be possible to obtain themon the fly. For example, the mBS may inform the fBS about itstransmission power during femtocell initialization. The fBS may thenestimate the path loss exponent from the mBS using the known distancebetween itself and the mBS. A position estimate of the location of thefBS may be obtained using any of several ways, such as GPS,triangulation, utilization of TV signals for position finding, or thea-priori information regarding the home's geographic location.⁴ See,also, the article “Co-Channel Femtocell Coverage Area Analysis”, byIsmail Guvenc, Moo-Ryong Jeong, and Fujio Watanabe, IEEE Commun Lett.,Volume 12, Issue 12, December 2008, Page(s): 880-882.

At step 40, when coverage area A_(fBS) is less than the threshold value,the fBS should choose one of the N−1 frequency bands that is differentthan F_(i). To minimize interference with other femtocells, thefemtocell preferably first scans the N−1 frequency bands. FIG. 6illustrates a spectrum scanning technique that may be used to implementstep 40, according to one embodiment of the present invention. As shownin FIG. 6, at step 42, the power levels in the N−1 frequency bands aredetermined using, for example, an energy detection technique. Then, atstep 44, the frequency band that has the least noise level is chosen.The chosen frequency band is then used for communication in thefemtocell (step 46). A large N provides a lesser inter-femtocellinterference, as there are more candidate frequency bands to choosefrom.

In addition to the coverage area A_(fBS) of a fBS, an approximate regionof coverage R_(fBS) may also be determined algebraically, as describedin further detail below. Frequently, region R_(fBS) may be approximatedby a circle, with the center of this circle being collinear with thelocations of the fBS and the mBS, and further away from the mBS relativeto the fBS. The distance between the center of the circle of regionR_(fBS) and the fBS location depends on such parameters as the path lossexponent and the transmission powers of the fBS and the mBS. Referringback to FIG. 4, the coverage region R_(fBS) for a given placement of anfBS provides a rough yardstick for determining if coverage area A_(fBS)completely covers the user's premises (step 60). If not, i.e., coveragearea A_(fBS) does not completely covers the user's premises, theplacement of the fBS may be adjusted to improve coverage (step 50).Steps 50 and 60 may be iterated to achieve an optimal placement of thefBS.

FIG. 5 is a flow chart illustrating another femtocell frequencyassignment method, according to a second embodiment of the presentinvention. At step 16, the respective geographical locations (x_(mBS),y_(mBS)) and (x_(fBS), y_(fBS)) of the mBS and the fBS are determined.At step 25, the distance d_(fBS) between the fBS and the mBS isdetermined from these geographical locations. Under this framework,knowledge of the path-loss exponents and the transmission powers of themBS and fBS are not required. Instead, the distance d_(fBS) is used todetermine if the fBS lies within or outside of the border (a thresholdvalue) between inner region 6 and outer region 7 of the macrocell (step35). If distance d_(fBS) is greater than the threshold value, thefemtocell uses frequency band F_(i) (step 70). Otherwise, at step 40(see details in FIG. 6), the femtocell selects the one of the other N−1frequency bands having the minimum noise level.

FIG. 7 shows yet another frequency band assignment framework, inaccordance with one embodiment of the present invention. As shown inFIG. 7, in a network where the macrocells have a reuse factor of N, aseparate frequency band F_(N+1) is assigned for use by all of the fBSswithin a macrocell. In this way, interference from the current mBS andany of the surrounding mBSs are avoided. However, because the fBSs usesa different frequency band from the serving mBS, hand-off process isless efficient.

According to yet another frequency band assignment framework, during aninitialization step of the femtocell, an MS or the femtocell fBS maymeasure the received signal strength and report the received signalstrength to the mBS. The mBS may then determine if the fBS belongs toinner region 6 or outer region 7, and report the determination to thefBS. Frequency band assignment may then proceed along any of the schemesdiscussed above based on the geographical region determination.

For an mBS located at x₁=(x₁; y₁), with transmit power P₁, and an fBSlocated at x₂=(x₂; y₂), with transmit power P₂, under an empirical pathloss model, the power of a signal transmitted from an mBS and receivedat an MS is given by

P _(r,1) =P ₁ P _(o)(d _(o) /d)^(n),  (1)

where P_(o) is the measured path loss at a reference distance d_(o)(typically, P_(o) may be approximated by (4π/λ)² for d_(o)=1, with λdenoting the wavelength of the signal) and n is the path loss exponent.Because the signal-to-interference ratio (SIR) is assumed to be 0 dB atthe border of a co-channel femtocell, without loss of generality, Po isthe same for both the fBS and the mBS at a distance d_(o)=1, so that:

$\begin{matrix}{\frac{P_{1}}{d_{1}^{n}} = {\frac{P_{2}}{d_{2}^{n}}.}} & (2)\end{matrix}$

Thus, in a system in which n=2, the coordinates x=(x; y) of the pointson the border of the femtocell satisfy:

d _(j) ²=(x−x _(j))²+(y−y _(j))² , jε{1, 2}.  (3)

Combining equations (3) and (2), one obtains:

$\begin{matrix}{{{x^{2} + y^{2} + {\frac{B_{1}}{A}x} + {\frac{B_{2}}{A}y} + \frac{C}{A}} = 0},} & (4)\end{matrix}$

where

A=P ₁ −P ₂,  (5)

B ₁=2P ₂ x ₁−2P ₁ x ₂,  (6)

B ₂=2P ₂ y ₁−2P ₁ y ₂,  (7)

C=+P ₁ x ₂ ² +P ₁ y ₂ ² −P ₂ x ₁ ² −P ₂ y ₁ ².  (8)

Equation (4) is in the form of a circle of radius r which is centered atk=(k₁; k₂), i.e.,

x ² +y ²−2k ₁ x−2k ₂ y+k ₁ ² +k ₂ ² −r ²=0.  (9)

Therefore, comparing equations (9) and (4), the center point k=(k₁; k₂),representing the center of femtocell coverage area, is

${k_{1} = {- \frac{B_{1}}{2A}}},{k_{2} = {- \frac{B_{2}}{2A}}},$

which may also be expressed in vector form as:

$\begin{matrix}{k = {{\frac{P_{2}}{P_{1} - P_{2}}x_{1}} - {\frac{P_{1}}{P_{1} - P_{2}}{x_{2}.}}}} & (10)\end{matrix}$

The radius r is given by:

$\begin{matrix}{r = {\sqrt{k_{1}^{2} + k_{2}^{2} - \frac{C}{A}} = {\sqrt{\frac{B_{1}^{2}}{4A^{2}} + \frac{B_{2}^{2}}{4A^{2}} - \frac{C}{A}}.}}} & (11)\end{matrix}$

Then, the femtocell coverage area is:

$\begin{matrix}{A_{fem} = {{\pi \; r^{2}} = {{\pi ( {\frac{B_{1}^{2}}{4A^{2}} + \frac{B_{2}^{2}}{4A^{2}} - \frac{C}{A}} )}.}}} & (12)\end{matrix}$

Equation (12) shows that the femtocell coverage area depends on both thelocations of the mBS and the fBS, and their respective transmit powers.

For n≠2 (i.e., for any arbitrary path loss exponent), taking the(2/n)-th root of both sides of equation (2) provides:

$\begin{matrix}{{\frac{P_{1}^{\frac{2}{n}}}{d_{1}^{2}} = \frac{P_{2}^{\frac{2}{n}}}{d_{2}^{2}}},} & (13)\end{matrix}$

which is equivalent to the scenario with n=2 above, but characterized bya different set of transmit powers. The center of coverage and the areaof coverage may be derived using the same procedure illustrated abovewith respect to equations (10) and (12), substituting (P₁)^(2/n) for P₁and (P₂)^(2/n) for P₂.

In general, the path loss exponents at the fBS and mBS are different(i.e., Po is not the same for both the fBS and the mBS at a distanced_(o)=1). In that case, rather than equation (2):

$\begin{matrix}{\frac{P_{1}}{d_{1}^{n_{1}}} = {\frac{P_{2}}{d_{2}^{n_{2}}}.}} & (14)\end{matrix}$

Because the mBS is typically located at a higher altitude (e.g., on acell tower), the power loss exponents may satisfy n₁<n₂. Taking the(2/n₂)-th root of both sides of equation (14), one obtains:

$\begin{matrix}{{\frac{P_{1}^{\frac{2}{n_{2}}}}{d_{1}^{2_{1}/n_{2}}} = {\frac{P_{2}^{\frac{2}{n_{2}}}}{d_{2}^{2}} = 0}},} & (15)\end{matrix}$

The assumption that the signal-to-interference ratio (SIR) is 0 dB atthe border of a co-channel femtocell results in:

P ₂ ^(2/n) ² (x ² +y ²−2xx ₁−2yy ₁ +x ₁ ² +y ₁ ²)^(n) ¹ ^(/n) ² −P ₁^(2/n) ² (x ² +y ²−2xx ₂−2yy ₂ +x ₂ ² +y ₂ ²)=0,  (16).

where 0<n₁/n₂<1. In equation (16), the highest power of both the x and yterms is 2. Equation (16) may be simplified by approximating d^(2n1/n2)and evaluating equation (16) at a point (a; b), using a second-orderTaylor series (Point (a;b) may be approximated by (x₁;x₂) in mostcases):

$\begin{matrix}{{{T( {x,y} )} = {{f( {a,b} )} + {( {x - a} ){f_{x}( {a,b} )}} + {( {y - b} ){f_{y}( {a,b} )}} + {0.5\lbrack {{( {x - a} )^{2}{f_{xx}( {a,b} )}} + {22( {x - a} )( {y - b} ){f_{xy}( {a,b} )}} + {( {y - b} )^{2}{f_{yy}( {a,b} )}}} \rbrack}}},} & (17) \\{\mspace{79mu} {{f( {a,b} )} = ( {a^{2} + b^{2} - {2{ax}_{1}} - {2{by}_{1}} + x_{1}^{2} + y_{1}^{2}} )^{\frac{n_{1}}{n_{2}}}}} & (18) \\{\mspace{79mu} {{f_{x}( {a,b} )} = {\frac{2n_{1}}{n_{2}}{( {a - x_{1}} )\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 1}}}} & (19) \\{\mspace{79mu} {{f_{y}( {a,b} )} = {\frac{2n_{1}}{n_{2}}{( {b - y_{1}} )\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 1}}}} & (20) \\{{f_{xx}( {a,b} )} = {{\frac{2n_{1}}{n_{2}}\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 1} + {\frac{4n_{1}}{n_{2}}( {\frac{n_{1}}{n_{2}} - 1} ){( {a - x_{1}} )^{2}\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 2}}}} & (21) \\{{f_{yy}( {a,b} )} = {{\frac{2n_{1}}{n_{2}}\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 1} + {\frac{4n_{1}}{n_{2}}( {\frac{n_{1}}{n_{2}} - 1} ){( {b - y_{1}} )^{2}\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 2}}}} & (22) \\{{f_{xy}( {a,b} )} = {\frac{4n_{1}}{n_{2}}( {\frac{n_{1}}{n_{2}} - 1} )( {a - x_{1}} ){{( {b - y_{1}} )\lbrack {f( {a,b} )} \rbrack}^{\frac{n_{1}}{n_{2}} - 2}.}}} & (23)\end{matrix}$

Then, using the result of equation (17) in equation (16), one obtains:

$\begin{matrix}{\mspace{79mu} {{{{\hat{A}}_{1}x^{2}} + {{\hat{A}}_{2}y^{2}} + {{\hat{B}}_{1}x} + {{\hat{B}}_{2}y} + {{\hat{B}}_{3}{xy}} + \hat{C}} = 0}} & (24) \\{\mspace{79mu} {{\hat{A}}_{1} = {{\frac{1}{2}P_{2}^{\frac{2}{n_{2}}}{f_{xx}( {a,b} )}} - P_{1}^{\frac{2}{n_{1}}}}}} & (25) \\{\mspace{79mu} {{\hat{A}}_{2} = {{\frac{1}{2}P_{2}^{\frac{2}{n_{2}}}{f_{yy}( {a,b} )}} - P_{1}^{\frac{2}{n_{1}}}}}} & (26) \\{\mspace{79mu} {{\overset{\sim}{B}}_{1} = {{P_{2}^{\frac{2}{n_{2}}}\lbrack {{f_{x}( {a,b} )} - {{af}_{xx}( {a,b} )} - {{bf}_{xy}( {a,b} )}} \rbrack} - {2P_{2}^{\frac{2}{n_{1}}}x_{2}}}}} & (27) \\{\mspace{79mu} {{\overset{\sim}{B}}_{2} = {{P_{2}^{\frac{2}{n_{2}}}\lbrack {{f_{y}( {a,b} )} - {{bf}_{yy}( {a,b} )} - {{af}_{xy}( {a,b} )}} \rbrack} - {2P_{1}^{\frac{2}{n_{1}}}y_{2}}}}} & (28) \\{\mspace{79mu} {{\overset{\sim}{B}}_{3} = {2P_{2}^{\frac{2}{n_{2}}}{f_{xy}( {a,b} )}}}} & (29) \\{\overset{\sim}{C} = {{P_{2}^{\frac{2}{n_{2}}}\lbrack {{f( {a,b} )} - {{af}_{x}( {a,b} )} - {{bf}_{y}( {a,b} )} + {0.5\; a^{2}{f_{xx}( {a,b} )}} + {0.5\; b^{2}{f_{yy}( {a,b} )}} + {{abf}_{xy}( {a,b} )}} \rbrack} - {{P_{1}^{\frac{2}{n_{1}}}( {x_{2}^{2} + y_{2}^{2}} )}.}}} & (30)\end{matrix}$

Equation (24) resembles the circle of equation (4), except for: 1) thexy cross-terms, so that the coverage area is an ellipse, rather than acircle, and 2) the coefficients for x₂ and y₂ are different. If P₂<<P₁one may set Ã₁≈Ã₂, and {tilde over (B)}₃≈0 (i.e., once againapproximating the coverage area by a circle). With this simplification,the coordinates at the center of the coverage area, and the area of thecoverage area, can be obtained using the procedures discussed above withrespect to equations (10) and (12), using equations (25)-(30).

Because spectrum resources are scarce, small frequency reuse factors arepreferable. Thus, a frequency reuse factor of N=1 may be preferable inmany future wireless systems. In such a system, femtocells may have touse the same frequency band as the macrocell in all locations within themacrocell coverage area. However, as discussed above, when an lBS isvery close to the mBS, the femtocell experiences severe interferencefrom the mBS. Interference averaging techniques, such as those discussedin Chandrasekhar (above) may be used to mitigate the interference. Suchtechniques have inherent disadvantages, and may also not be sufficientto overcome the interference in femtocells at close proximities to themBS.

FIG. 8 shows, for N=1, another frequency assignment scheme which assignsa separate frequency band F₂ to all the femtocells in a macrocellnetwork using frequency band F₁, according to one embodiment of thepresent invention. The system of FIG. 8 ensures interference-freeoperation among the femtocells and the macrocell, at the expense ofadditional spectrum resources. Also, when available, more than onefrequency band may be assigned for femtocell operations to reduceinter-femtocell interference, using a spectrum-sensing approach, such asdescribed for step 40 of FIGS. 4-6.

The scenarios discussed above are applicable to downlink channelassignments. The interference for the uplink may be different, and adifferent channel assignment scheme may be needed for duplex operation.For example, a macrocell MS (mMS) may need a larger transmit power toreach the mBS, when the MS is far away from the mBS. Hence, when thefemtocell is located within outer region 7, as far as the uplinkoperation is concerned, the interference from an mMS to the femtocellmay be more significant, when the femtocell and the mMS both use thesame channels for communication. Thus, to avoid interference between themMS and the femtocell, different channels are preferably assigned to themMS and femtocell in outer region 7. Within inner region 6, because themMS uses weaker signals to communicate with the mBS, co-channeloperation with the femtocell may be possible.

FIG. 9 shows a frequency division duplex (FDD) frequency assignmentscheme for a macrocell network, in accordance with one embodiment of thepresent invention. As shown in FIG. 9, the macrocell uses frequency band0 (i.e., frequency band F_(i)) in the downlink, and frequency band 12(i.e., frequency band F_(N+i)) in the uplink. In such a network, thefrequency band assigned to the femtocell depends on both 1) whether ornot the femtocell is within inner region 6 or outer region 7, and 2)whether or not the communication is downlink or uplink. FIG. 10 shows anexample of an actual FDD frequency assignment in a network of the typediscussed with respect to FIG. 9, according to one embodiment of thepresent invention. The downlink frequency assignment may be achievedusing the methods in FIGS. 4-8. For the uplink, when the femtocell iswithin inner region 6, the same frequency band is used for the femtocellas the macrocell (i.e., frequency band F_(N+i)). Otherwise, i.e., if thefemtocell is in outer region 7, a different frequency band thanfrequency band F_(N+i) is used to avoid interference to or from the mMSthat are far away from the mBS.

According to another aspect of the present invention, power control andfrequency assignment may be carried out simultaneously (“joint powercontrol and frequency assignment”). The macrocell network of FIG. 9above may be used to illustrate such an approach, for the case where thefrequency reuse factor of N>1. In such a network, N downlink frequencybands and N uplink frequency bands are used, with each frequency bandbeing assigned to a different macrocell. In the following detaileddescription, downlink operation is first discussed. Without loss ofgenerality, frequency band 0 (i.e., frequency band F_(i)) is assigned tothe macrocell of interest. For N=3, which is illustrated above in FIG.2, where no neighboring macrocells use the same frequency band, and themacrocell of interest uses frequency band 4 (i.e., frequency band F₁).As explained above, within each frequency band, users may also befurther separated in time, frequency, or code domains. For example, in awideband code division multiple access (WCDMA) system, multiple CDMAcodes are used to minimize interference among the different users withina macrocell.

To provide joint power control and frequency assignment, a methodaccording to the present invention follows the following criteria: (a)co-channel operation of the femtocell is preferable from cell-search andhand-off points of view, subject to the interference conditions from themBS; (b) in all cases, the femtocell guarantees a minimum coverage areaA_(fem) through power control; and (c) the femtocell has a maximumtransmission power limit P_(max) and a minimum transmission power limitP_(min). Power limit P_(max) may represent hardware constraints orinterference constraints among femtocells, while power limit P_(min) maybe the minimum transmission power needed for good coverage, in theabsence of any interference. FIG. 11 illustrates a network implementingjoint power control and frequency assignment, in accordance with oneembodiment of the present invention. As shown in FIG. 11, the macrocellis partitioned into inner region 111, power control region 112, andouter region 113. While no power control is applied in inner and outerregions, the fBS uses power control in power-control region 112 toprovide a pre-determined femtocell coverage area A_(fem).

As discussed above, the coverage area of the IBS is determined by thearea within a contour along which the received power levels (typically,for the pilot signals, rather than the data signals) from the fBS andmBS are the same. The present invention, however, is equally applicableto systems in which the coverage areas are defined in other ways, suchas hand-off parameters. Such hand-off parameters may include, forexample, cell search initiation threshold and handoff execuationthreshold. In such cases, the received power levels from the fBS and mBSmay not be the same, and there may be more than one contour that definesthe coverage area (e.g., one contour for an incoming user to the fBS andanother contour for an outgoing user from the IBS).

FIG. 12 provides an example of frequency band assignments in the regionsof FIG. 11, for both downlink and uplink frequency assignments, inaccordance with one embodiment of the present invention. As shown inFIG. 12, during downlink transmission, macrocell uses frequency bandF_(i) in all three regions. The frequency band used for downlink by afemtocell, however, depends on which region the fBS is located: (a) whenthe femtocell is in power control region 112, or in outer region 113,the femtocell uses the same frequency band as the macrocell (i.e.,F_(i).); and (b) when the femtocell is within inner region 111 (i.e.,when interference from the mBS is high), the fBS uses a frequency banddifferent than that used by the macrocell (e.g., a frequency band usedby one of the neighboring macrocells, or a frequency band specificallyreserved for femtocells).

FIG. 13 is a block diagram illustrating joint power control andfrequency band assignment in the regions of FIG. 11, in accordance withone embodiment of the present invention. In FIG. 13, if the locations ofthe fBS and the mBS are available (step 124), a fBS obtains at step 110(a) n₁ and n₂, which denote the path loss exponents for the mBS and thefBS, respectively; (b) P₁ and P₂, which denote the transmission powersfor the mBS and fBS, respectively; and (c) (x_(mBS),y_(mBS)) and(x_(fBS),y_(fBS)), which denote the locations of the mBS and the fBS,respectively. Otherwise, i.e., the locations of the fBS and the mBS arenot available, distance d_(fm) between an fBS and mBS is obtained atstep 126. At step 120, using the information obtained in either step 126or step 110, the fBS determines if it is in inner region 111, powercontrol region 112, or outer region 113. If the fBS is in power controlregion 112, at step 60, the femtocell is assigned the same frequencyband as the macrocell (i.e., frequency band F_(i) for downlinkoperations). In power control region 112, at step 190, the fBS selects apower level P₂ which ensures that coverage area A_(fem) is achieved, inthe presence of macrocell interference. If the fBS determines that it isin outer region 113, at step 170, the same frequency band as themacrocell is assigned to femtocell (i.e., frequency band F_(i) fordownlink operations) (i.e., frequency band F_(i) for downlinkoperations). In outer region 113, even though interference by the mBS tothe femtocell is low, a low transmission power may result inunsatisfactory performance. Therefore, at step 1100, a fixed minimumtransmission power P_(min) is selected for the fBS. If the femtocell isdetermined to be in neither power control region 112, nor outer region113 (i.e., fBS is in inner region 111), at step 150, a frequency bandwhich is orthogonal to the macrocell frequency band is assigned to thefemtocell. Under such an assignment, interference from the macrocell isinsignificant. Thus, in inner region 111, the femtocell may also selectat step 1110 a fixed transmission power P_(min) for communication. Alarger fixed transmission power may be used to overcome interferencefrom neighboring macrocells or other femtocells.

FIGS. 14 and 15 are block diagrams illustrating different approaches thefBS may use to decide the region of its location (i.e., step 120 of FIG.13), in accordance with one embodiment of the present invention. In FIG.14, at step 131, a femtocell calculates a transmission power P₂ requiredto obtain a coverage area A_(fem). Some examples of such a calculationis discussed in further detail below. Then, at step 132, the calculatedpower level P₂ is compared to power thresholds P_(min) and P_(max)discussed above. If P_(min)<P₂<P_(max) (step 135), the femtocell is inpower control region 112. Otherwise, if P₂>P_(max) (step 133), thefemtocell is in inner region 111 (step 136), and if P₂<P_(min) (step134), the femtocell is in outer region 113 (step 137). In the method ofFIG. 14, the power level at step 131 may be calculated using theparameters n₁, n₂, P₁, A_(fem), and the locations of the fBS and mBS(i.e., step 110 of FIG. 13), or the distance between the fBS and the mBS(i.e., step 126 of FIG. 13) and other remaining parameters.

FIG. 15 shows a simpler approach by which the femtocell can determinethe region of its location. In the method of FIG. 15, only distanced_(fin) to the mBS, calculated at step 141, is used to decide whether itis in inner region 111, power control region 112, or outer region 113(i.e., the parameters n₁, n₂, P₁, A_(fem) are not utilized). At step142, if d_(in)<d_(fin)<d_(out), where d_(in) and d_(out), are the radiiof inner region 111 and outer region 13, respectively, the femtocell isdetermined to be in power control region 112 (step 145). Otherwise, ifd_(fin)>d_(out) (step 143), the femtocell is determined to be in outerregion 113, while if d_(fin)<d_(in) (step 144), the femtocell isdetermined to be in inner region 111 (step 147).

In one implementation of a method of the present invention, where thecoverage area A_(fem) may not be available, but the total coverage areaof a uses premises may be classified into K premise types (e.g., astudio, small apartment, large apartment, house, or office), apredetermined coverage area may be assigned for the premises for thepurpose of calculating the transmit power, according to the premisetype.

FIG. 16 shows one implementation of a method for refining thecalculation of transmit power, in accordance with one embodiment of thepresent invention. As shown in FIG. 16, at step 152, an initialtransmission power level P₂ of the fBS is determined using any method,such as the method of FIG. 14. Then, transmission power level P₂ isupdated at step 154 by the responses to periodic control signals sent tothe MSs. By taking short-term and long-term averages of the receivedsignal powers at the fBS and at the MSs, average signal-to-noise ratios(SNRs) of different MSs may be obtained (step 156). Then, at step 158,the fBS may tune the transmission power level P₂ to achieve anacceptable receiver SNR. In other words, based on received signals, thefBS may increase or decrease its transmission power around the initialpower estimate to satisfy different SNR metrics.

According to the present invention, various criteria based on SNR may beused to set the power level at the fBS. These criteria may include (a)average SNR at the fBS, (b) average SNR at the MSs, (c) average SNR atthe fBS and the MSs, (d) minimum SNRs at the fBS (for different timescales), (e) minimum SNRs at the MSs (at different time scales), and (f)minimum SNRs at the fBS and minimum SNRs at the MSs (at different timescales). The received signals may show smaller variations for a smallapartment, so that short-term averages would provide information fornecessary power levels, while typically much larger variations areexpected for a large house, thereby necessitating long-term averages.

Apart from the received SNR, the fBS may utilize some other metrics forsetting its transmission power at step 156 of FIG. 16. For example, ifno response is received from any MS to the control messages transmittedfrom the fBS for a certain period of time, the fBS may conclude thatthere is no MS to communicate with the fBS at that time. Under suchcondition, the fBS may set its (pilot) transmission power to a minimumlevel (i.e., P_(min)) to minimize interference with other femtocellsand/or the macrocell. Also, the fBS may record the activities of themMSs at different time scales to adjust its transmission power. Forexample, between the morning and evening, there may be little connectionfrom MSs to a femtocell, and considerably more activities may occur inthe evening. By monitoring such usage patterns, an fBS may decide tominimize its transmission power during the daytime.

FIG. 17 shows an example message exchange between an fBS and an MS, inaccordance with one embodiment of the present invention. As shown inFIG. 17, the fBS sends control message 162 initially. If no response isreceived from any MS to control message 162 after a time period, at step172, the fBS may set its transmission power to a minimum level tominimize interference to other femtocells and the macrocell. However, ifthe fBS receives a response 164 from an MS, the fBS may utilize thissignal to estimate the SNR of the MS. Once having SNRs calculated fromcommunications with different MSs, the fBS may update at step 174 itstransmission power based on the SNRs for future communications (step176). The fBS may transmit control messages periodically (step 178) toadaptively update its transmission power, in response to changes in theenvironment (e.g., number of users, locations of users, or channelqualities of the users).

The transmission power level may be determined using, for example, thefollowing method. From equations (4)-(8) above, in a femtocellinterference-limited coverage area (ILCA) environment with equal pathloss exponents, circular coverage area A_(fem) is given by

$\begin{matrix}{A_{fem} = {{\pi \; r^{2}} = {\pi ( {\frac{B_{1}^{2}}{4\; A^{2}} + \frac{B_{2}^{2}}{4A^{2}} - \frac{C}{A}} )}}} & (31)\end{matrix}$

In a power-controlled femtocell environment, the femtocell provides acoverage area A_(fem) at all times by adjusting its transmission powerP₂ (which is calculated for a given A_(fem)). Equation (31) may berearranged to:

$\begin{matrix}{{B_{1}^{2} + B_{2}^{2} - {4{AC}} - \frac{4A^{2}A_{fem}}{\pi}} = 0} & (32)\end{matrix}$

which may be simplified to:

$\begin{matrix}{{{{aP}_{2}^{2} + {bP}_{2} + c} = 0}{Where}} & (33) \\{{a = \frac{{- 4}A_{fem}}{\pi}}{b = {4{P_{1}( {( {x_{1} - x_{2}} )^{2} + ( {y_{1} - y_{2}} )^{2} + \frac{2A_{fem}}{\pi}} )}}}{c = \frac{{- {rP}_{1}^{2}}A_{fem}}{\pi}}} & (33)\end{matrix}$

The required power level P₂ that provides coverage area A_(fem) may besolved by finding the roots of the second order polynomial of equation(32). Equation (13) above relates the transmit power levels P₁ and P₂ tothe coverage areas of the mBS and the fBS, respective, for a network inwhich n≧2, n being any arbitrary path loss exponent. As discussed above,equation (13) represents the case where n=2, but with a different pairof transmit powers. The techniques above may be used to calculate therequired transmit power P₁.

The above detailed description is provided to illustrate the specificembodiments of the present invention, and is not intended to belimiting. Numerous variations and modifications are possible within thescope of the present invention. The present invention is set forth inthe following claims.

1. A method for selecting a frequency band for use in a femtocelllocated within a coverage area of a macrocell, comprising: partitioningthe coverage area of the macrocell into an inner region and an outerregion; selecting for the femtocell a frequency band used in themacrocell, when the femtocell is located within the outer region, andselecting for the femtocell a different frequency band than a frequencyband used in the macrocell, when the femtocell is located within theinner region, wherein a mobile station switches between the macrocelland the femtocell after a cell-search step.
 2. A method as in claim 1,wherein the macrocell neighbors a plurality of other macrocells, andwherein the different frequency band selected for the femtocell is alsodifferent from any frequency band used in such other macrocells.
 3. Amethod as in claim 1, wherein the different frequency band is selectedusing a spectrum-sensing technique.
 4. A method as in claim 3, whereinthe different frequency band is selected in a manner that reducesinter-femtocell interference.
 5. A method as in claim 1, wherein thedifferent frequency band is selected so as to maintain a femtocellcoverage area greater than a threshold value.
 6. A method as in claim 1,further comprising determining both a coverage area and a coverageregion for the femtocell, and based on the determined coverage area andthe determined coverage region, positioning a base station of thefemtocell within the coverage region.
 7. A method as in claim 6, whereinthe coverage region covers a predetermined premise.
 8. A method as inclaim 1, wherein the selected frequency band is selected for a downlink,and wherein a frequency assignment for an uplink is based oninterference from one or more mobile stations in the macrocell.
 9. Amethod as in claim 1, wherein a base station in the femtocell measures asignal quality in a received signal and reports the signal quality to abase station in the macrocell, and wherein the base station of themacrocell determines if the femtocell is within the inner region or theouter region.
 10. A method as in claim 1, wherein the partitioningfurther partitions the coverage area of the macrocell into a powercontrol region.
 11. A communication system, comprising: a macrocellhaving a coverage area partitioned into an inner region and an outerregion; a first femtocell within the outer region, the first femtocellcommunicating using a frequency band used in the macrocell; and a secondfemtocell within the inner region, the second femtocell communicatingusing a different frequency band than a frequency band used in themacrocell.
 12. A communication system as in claim 11, wherein thecoverage area is partitioned using a distance from a base station of themacrocell.
 13. A communication system as in claim 11, further comprisinga mobile station that switches between the macrocell and one of thefemtocells after a cell-search step.
 14. A communication system as inclaim 11, further comprising a mobile station that switches between themacrocell and one of the femtocells after a hand-off step.
 15. Acommunication system as in claim 11, further comprising a plurality ofother macrocells neighboring the macrocell, and wherein the differentfrequency band selected for the femtocell is also different from anyfrequency band used in such other macrocells.
 16. A communication systemas in claim 11, wherein the different frequency band is selected using aspectrum-sensing technique.
 17. A communication system as in claim 16,wherein the different frequency band is selected in a manner thatreduces inter-femtocell interference.
 18. A communication system as inclaim 16, wherein the different frequency band is selected so as tomaintain a femtocell coverage area greater than a threshold value.
 19. Acommunication system as in claim 16, further comprising a base stationof one of the femtocells that is placed at a position determined basedon a coverage area and a coverage region determined for the femtocell ofthe base station.
 20. A communication system as in claim 19, wherein thecoverage region covers a predetermined premise.
 21. A communicationsystem as in claim 11, wherein the selected frequency band is selectedfor a downlink, and wherein a frequency assignment for an uplink isbased on interference from one or more mobile stations in the macrocell.22. A communication system as in claim 11, wherein a base station in oneof the femtocells measures a signal quality in a received signal andreports the signal quality to a base station in the macrocell, andwherein the base station of the macrocell determines if the femtocell iswithin the inner region or the outer region.